Reader's Note:
Science and medicine have a deep and proven understanding of sex. There is no way for any human being to change sex. Even nearly every single person with rare conditions that are considered differences of sex development conditions can be reliably sorted into the correct sex category.
This is done using a combination of aspects of the person's body and ultimately it is whether the body is formed to produce small or large gametes. And this is regardless of whether the body successfully has, is or will in the future produce those small or large gametes. The body will not have working ovaries and testes at the same time.
This argument about 'no one knows their chromosomes' or 'it is no one's business' is simply thought terminating phrases used to distract from the reality. Generally, by puberty each human being will know whether they are male or female. If there is an issue, it will generally be discovered at that time. Sometimes, it is later. It is also very rare that there is an issue. There is a minuscule number of people with a DSD that are also trans.
Human beings can also fairly reliably over a short time correctly sex a male person who has not received puberty blockers. And sometimes even then. It may be more difficult with extreme facial surgery, where a face is peeled all the way back and all the male cue bones are shaved, cheek implants added, the area between the nose and the lip shortened, the hair line changed etc. Even then, it is not necessarily successful in real life. However, there is always, always the gait, the skeletal proportions and other variables including voice that remain identifiable cues. Female humans have a higher success rate in removing their female cues from their body.
Yet, extreme trans activists will utilise one group's unique medical condition to try to convince people that sex is not reliably stable and immutable among humans. And that particularly women and children cannot correctly sex a male person with high accuracy in real life interaction.
But also, as I and others have pointed out, there ARE times when it IS another person's business as to what sex another human being is. To dismiss this is ultimately harmful and it is ignorant.
The 'I don't understand why posters here think it's so important' is because this poster refuses to engage with the countless explanations across different threads now. This is determined ignorance at this point, whether this poster intends it or not.